Introduction

Compacting a Coal Fly Ash Stabilized Base.

Stabilized base or subbase layers are pavement layers composed of a compacted mixture of aggregate and cementitious material. The binder material is usually lime or cement, though additional pozzolanic materials may also be added. For new construction, the base or subbase materials are mixed with the binder and water if needed, either in place or at a plant, and are then graded and compacted. Stabilized base layers can also be formed through full depth reclamation when binder is added to the reclaimed pavement material. Stabilized layers provide a strong foundation for both rigid and flexible pavements, though stabilized pavement layers are usually used in flexible pavements. It should be noted that calcium chloride is also used for stabilization in the Southern states, and asphalt cement and asphalt emulsions have also been used as well. However, the focus of this document will be on lime and cement based stabilization.

IRC Materials in Stabilized Base and Subbase

IRC materials have been used successfully in stabilized base and subbase applications as the binder, the pozzolanic material admixture or as both the fine and coarse aggregate. Class F coal fly ash in particular is very commonly used as a pozzolanic admixture in combination with lime or with cement. Class C coal fly ash can be used by itself as the binder because of its cementitious properties. Coal bottom ash and boiler slag can also be used as the fine aggregate or even to meet the entire aggregate requirement for base and subbase mixtures. Slag materials have also been used in stabilized layers. Slag cement can be used as the binder for stabilized base and subbase layers, while air cooled blast furnace slag has been used very successfully as fine aggregate. Other IRC materials that have been used successfully in stabilized base applications include foundry sands, which have been used as a fine aggregate and crushed concrete, which is used for both coarse and fine aggregate. One benefit of recycling concrete is that it keeps high quality natural aggregates in use. In addition, Portland cement concrete pavements can be recycled on site, which reduces project costs by eliminating the transportation costs associated with removing the old concrete.

PCC pavements can be recycled on site, with
reclaimed concrete aggregate used as base.

Benefits

The use of IRC materials as aggregates in high volume applications like base and subbase layers reduces the need for mining virgin aggregate and the associated use of water, fuel and reduces carbon dioxide emissions, while also saving valuable landfill space. At the same time, the performance of these materials is as good or better then natural materials, which provides added value to the project because of the reduced costs. In addition, the use of coal fly ash and slag cement provides strong, cost effective pavement layers while reducing green house gas emissions and energy consumption compared to cement.